Abstract: Objective To evaluate the clinical effects of the crown lengthening surgery on the restoration of posterior residual roots and crowns with subgingivally involved defects. Methods From December 2009 to December 2011, according to the subgingival depths of the defects, 50 teeth from 48 patients with subgingivally involved defects were divided into three groups: group A(depth<3.0 mm),group B (3.0 mm ≤depth≤4.0 mm), group C(depth>4.0 mm). All teeth were treated by the crown lengthening first and then underwent post-crown prosthesis 6 weeks after operation. PD, SDI, and lengths from gingival margin to the frature ends of teeth before operation and 6 weeks after operation were recored and compared. Results The teeth sections of AB groups were exposed preferably and the periodontal health was good. The effective rate was 100%. 60% of C group were exposed well and were repaired well. There was significant difference between A and C groups, as well as B and C groups (P<0.05) . Postoperative PD,and SBI were better than those preoperative values(P<0.05). There was no differences between A and B groups (P>0.05), while there was significant differences between group C and groups A and B (P <0.05). Conclusion Crown lengthening surgery is a good method for the protection and restoration of posterior residual roots and crowns, but dentists should pay attention to the selection of the indications.